Sum Root to Leaf Numbers

Given a binary tree containing digits from 0-9 only, each root-to-leaf path could represent a number.

An example is the root-to-leaf path 1->2->3 which represents the number 123.

Find the total sum of all root-to-leaf numbers.

For example,

    1
   / \
  2   3
The root-to-leaf path 1->2 represents the number 12.
The root-to-leaf path 1->3 represents the number 13.

Return the sum = 12 + 13 = 25.

计算所有路径的和，从根节点到叶子节点每条路径都代表一个十进制的数字。用递归从根节点开始，依次记录所有路径上组成数字的和。代码如下：

/**
 * Definition for a binary tree node.
 * public class TreeNode {
 *     int val;
 *     TreeNode left;
 *     TreeNode right;
 *     TreeNode(int x) { val = x; }
 * }
 */
public class Solution {
    public int sumNumbers(TreeNode root) {
        if(root == null) return 0;
        return sum(root, root.val);
    }
    public int sum(TreeNode root, int value) {
        if(root == null) return value;
        if(root.left != null && root.right != null) 
            return  sum(root.left, value * 10 +root.left.val) 
            + sum(root.right, value * 10 +  root.right.val);
        if(root.left != null) 
            return sum(root.left, value * 10 +root.left.val);
        if(root.right != null) 
            return sum(root.right, value * 10 +  root.right.val);
        return value;
    }
}